A centrifugal clutch typically includes a driver or impeller configured so as to be rotated about an axis by an external power or driving source, a hollow drum coaxial with and disposed about the driver and configured to drive an external load, and one or more shoes located within the drum that are rotated relative to the drum by the driver. One such clutch is disclosed in U.S. Pat. No. 6,857,515, which is incorporated herein by reference in its entirety.
The shoes are generally adapted to move radially into and out of frictional engagement with the inside of the drum. The frictional engagement with the drum provides load transmission between the driver and the drum. The shoes are typically urged towards the center, away from the drum, by one or more springs. As the driver and the shoes rotate about the axis, the centrifugal force created by the rotation urges the shoes radially outward. When the speed of rotation is sufficiently high, the centrifugal force acting on the shoes overcomes the force of the springs, urging the shoes to move outward to engage the drum. The engagement of the shoes with the drum causes the drum, and thus, the external load, to rotate in combination with the shoes. The speed at which the clutch engages is, therefore, determined by a balance between the mass of the shoes and the strength of the springs.
Centrifugal clutches are commonly used in the drive trains of machines powered by small internal combustion engines for producing varying amounts of horsepower. These types of clutches have particular use in lower horsepower machines, such as wood chippers and go-karts, which typically operate at up to about 40 hp. Particularly with regard to racing go-karts, there are several benefits to having a readily adjustable clutch. Each racetrack may be different due to the length of the track, the material from which the track is composed, the banking of the turns, the radius of the turns, and the temperature and dampness conditions on the track at the time of racing. All of these factors may require tuning or adjustment of a go-kart clutch for optimum performance, so that the engine can be maintained in the power band. Additionally, track conditions may change throughout the race, due to changes in temperature and weather. Therefore, it is apparent that a clutch capable of being easily and quickly adjustable would advantageous for go-kart racing.
The clutch is typically set to disengage when the engine is idling, and to engage when the engine is generating sufficient torque to drive the load effectively. Because the engagement between the shoes and the drum is based on friction, a certain amount of slippage is inherent, and in some cases is actually required, in the operation of the clutch. For example, when the centrifugal force first overcomes the spring force, the initial contact between the moving shoe and the stationary drum will result in slippage. As the speed of the motor increases, the centrifugal force should eventually produce sufficient friction to prevent slippage. Until that speed is attained, there will be relative movement between the shoes and the drum. This slippage is necessary to some degree to provide for a gradual acceleration of the driven component.
In some applications, it is desirable to have a clutch whose performance characteristics can be readily adjusted depending on operating conditions. The performance characteristics of a clutch, including the speed at which the shoes engage the drum, the amount of torque that can be transmitted between the shoes and the drum, and the character of initial engagement and subsequent slippage between the shoes and the drum, is dependent on several factors. These factors may include the mass of the shoes, the strength of the springs, the geometry of the shoes, and the mass distribution within each shoe. In one example, the use of heavier shoes and lighter springs will generally result a lower engagement speed and a higher torque capacity with less slippage at the same external load. In another example, a shoe having a geometry or density whereby its mass is biased towards its leading edge and away from its trailing edge will generally result in a more aggressive engagement of the clutch that may provide a higher torque capacity with less slippage. It can be appreciated that various combinations of these several factors may be adjusted to fine-tune the performance of a clutch to a particular application under particular operating conditions.